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Label Switched Path (LSP) Ping/Trace over MPLS Network using Entropy Labels (EL)
Cisco Systemsnobo@cisco.comCisco Systemsswallow@cisco.comCisco Systemscpignata@cisco.comMPLS Working Group
Internet Engineering Task ForceMPLSLSP PingEntropyThe Multiprotocol Label Switching (MPLS) Label Switched Path
(LSP) Ping and Traceroute are used to exercise specific paths of
Equal Cost Multipath (ECMP). This ability has been lost on some
scenarios which makes use of : Entropy
Labels (EL).
This document extends the MPLS LSP Ping and Traceroute
mechanisms to restore the ability of exercising specific paths
of ECMP over LSP which make use of Entropy Label. This
document updates and
.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.Section 3.3.1 of specifies
multipath information encoding which can be used by LSP Ping
initiator to trace and validate all ECMP paths between ingress
and egress. These encodings are sufficient when all the LSRs along
the path(s), between ingress and egress, consider same set of
"keys" as input for load balancing algorithm: all IP
based or all label based.With introduction of , it is quite
normal to see set of LSRs performing load balancing based on
EL/ELI while others still follow the traditional way (IP
based). This results in LSP Ping initiator not be able to
trace and validate all ECMP paths in following scenarios:
One or more transit LSRs along ELI/EL imposed LSP do not
perform ECMP load balancing based on EL (hashes based on
"keys" including IP destination address). This
scenario is not only possible but quite common due transit
LSRs not implementing or transit
LSRs implementing but not
implementing suggested transit LSR behavior in Section 4.3
of .Two or more LSPs stitched together with at least one LSP
being ELI/EL imposing LSP. Such scenarios are described in
.
These scenarios will be quite common because every deployment
of will invariably end up with nodes
that support ELI/EL and nodes that do not. There will
typically be areas that support ELI/EL and areas that do
not.As pointed out in the procedures of
with respect to multipath
information type {9} are incomplete. However does
not actually update . Further the specific EL location
is not clearly defined, particularly in the case of FAT
Pseudowires . Herein is defined a
new FEC Stack sub-TLV for the Entropy Label.
of this document updates the
procedures for multipath information type {9}.
describes LSP traceroute as an
operation performed through initiating LSR sending LSP Ping
packet (LSP echo request) with incrementing TTL, starting with
TTL of one. Initiating LSR discovers and exercises ECMP by
obtaining multipath information from each transit LSR and using
specific destination IP address or specific entropy label.LSP Ping initiating LSR sends LSP echo request with multipath
information. This multipath information is described in
DSMAP/DDMAP TLV of echo request, and can contain set of IP
addresses or set of labels today. Multipath information types
{2, 4, 8} carry set of IP addresses and multipath information
type {9} carries set of labels. Responder LSR (receiver of LSP
echo request) is to determine subset of initiator specified
multipath information which load balances to each downstream
(outgoing interface). Responder LSR sends LSP echo reply with
resulting multipath information per downstream (outgoing
interface) back to the initiating LSR. Initiating LSR is then able
to use specific IP destination address or specific label to
exercise specific ECMP path on the responder LSR.Current behavior is problematic in following scenarios:
Initiating LSR sends IP multipath information, but
responder LSR load balances on labels.Initiating LSR sends label multipath information, but
responder LSR load balances on IP addresses.Initiating LSR sends any of existing multipath information
to ELI/EL imposing LSR, but initiating LSR can only
continue to discover and exercise specific path of ECMP if
ELI/EL imposing LSR responds with both IP addresses and
associated EL corresponding to each IP address. This is
because:
ELI/EL imposing LSR that is a stitching point will
load balance based on IP address.Downstream LSR(s) of ELI/EL imposing LSR may load
balance based on ELs.Initiating LSR sends any of existing multipath information
to ELI/EL imposing LSR, but initiating LSR can only
continue to discover and exercise specific path of ECMP
if ELI/EL imposing LSR responds with both labels and
associated EL corresponding to label. This is because:
ELI/EL imposing LSR that is a stitching point will
load balance based on EL from previous LSP and imposes
new EL.Downstream LSR(s) of ELI/EL imposing LSR may load
balance based on new ELs.
The above scenarios point to how the existing multipath
information is insufficient when LSP traceroute is operated on
an LSP with Entropy Labels described by . Therefore, this document defines a multipath information
type to be used in the DSMAP/DDMAP of LSP echo request/reply
packets in .
In addition, responder LSR can reply with empty multipath
information if no IP address set or label set from received
multipath information matched load balancing to a
downstream. Empty return is also possible if initiating LSR sends
multipath information of one type, IP address or label, but
responder LSR load balances on the other type. To disambiguate
between the two results, this document introduces new flags in
the DSMAP/DDMAP TLV to allow responder LSR to describe the load
balance technique being used.It is required that all LSRs along the LSP understand new
flags as well as new multipath information type. It is also
required that initiating LSR can select both IP destination
address and label to use on transmitting LSP echo request
packets. Two additional DS Flags are defined for the DSMAP and
DDMAP TLVs in .
defined multipath type {9} for tracing of LSPs where
label based load-balancing is used. However, as pointed out
in , the procedures for using this type are incomplete.
First, the specific location of the label was not defined.
What was assumed, but not spelled out, was that the presence
of multipath type {9} meant the responder
should act as if the payload of the received packet were
non-IP and that the bottom-of-stack label should be replaced
by the values indicated by multipath type {9} to determine
their respective out-going interfaces.
Further, with the introduction of , entropy labels may
now appear anywhere in a label stack.
This section defines to which labels multipath type {9} can
apply. Additionally it defines procedures for tracing
pseudowires and flow-aware pseudowires. These procedures
pertain to the use of multipath information type {9} as well
as type {10}.
defines a new FEC-Stack sub-TLV to
indicate and entropy label. Multipath type {9} applies
exclusively to this sub-TLV. Any LSP Ping message containing
a DD-MAP or DS-MAP with multipath type {9} MUST include an
EL_FEC at the bottom of the FEC-Stack.
When an MPLS echo request message is received containing a
FEC-Stack with an EL-FEC at the bottom of the FEC stack and is
not preceded by an entropy label, the responder must behave
(for load balancing purposes) as if the first word of the
message were a Pseudowire Control Word.
In order to trace a non-FAT pseudowire, instead of
including the appropriate PW-FEC in the FEC-Stack, an EL-FEC
is included. Tracing in this way will cause compliant routers
to return the proper outgoing interface. Note that this
procedure only traces to the end of the MPLS transport LSP
(e.g. LDP and/or RSVP). To actually verify the PW-FEC or in
the case of a MS-PW, to determine the next pseudowire label
value, the initiator MUST repeat that step of the trace, (i.e.,
repeating the TTL value used) but with the FEC-Stack
modified to contain the appropriate PW-FEC.
In order to trace a FAT pseudowire, the initiator
includes an EL-FEC at the bottom of the FEC-Stack and pushes
the appropriate PW-FEC onto the FEC-Stack.
In order to facilitate the flow of the following text we
speak in terms of a boolean called EL_LSP maintained by the
initiating LSR. This value controls the multipath information type to be
used in transmitted echo request packets. When the initiating LSR is
transmitting an echo request packet with DSMAP/DDMAP with a non-zero
multipath information type, then EL_LSP boolean MUST be consulted to
determine the multipath information type to use.
In addition to procedures described in as updated by and , initiating LSR MUST operate
with following procedures.
When initiating LSR is IP based load balancer (not imposing ELI/EL), initialize EL_LSP=False.When initiating LSR imposes ELI/EL, initialize
EL_LSP=True.When initiating LSR is transmitting non-zero multipath
information type:
If (EL_LSP) initiating LSR MUST use multipath
information type {10}.Else initiating LSR MUST use multipath
information type {2, 4, 8, 9}.When initiating LSR is transmitting multipath information
type {10}, both "IP Multipath Information" and
"Label Multipath Information" MUST be included,
and "IP Associated Label Multipath Information"
MUST be omitted (NULL).When initiating LSR receives echo reply with {L=0, E=1} in
DS flags with valid contents, set EL_LSP=True.In following conditions, initiating LSR may have lost the
ability to exercise specific ECMP paths. Initiating LSR MAY
continue with "best effort".
Received echo reply contains empty multipath
information.Received echo reply contains {L=0, E=<any>} DS
flags, but does not contain IP multipath information.Received echo reply contains {L=1, E=<any>} DS
flags, but does not contain label multipath information.Received echo reply contains {L=<any>, E=1} DS
flags, but does not contain associated label multipath
information.IP multipath information types {2, 4, 8} sent, and
received echo reply with {L=1, E=0} in DS flags.Multipath information type {10} sent, and received echo
reply with multipath information type other than {10}.Common Procedures: Responder LSR receiving LSP echo request
packet with multipath information type {10} MUST validate
following contents. Any deviation MUST result in responder LSR
to consider the packet as malformed and return code 1
(Malformed echo request received) in LSP echo reply packet.
IP multipath information MUST be included.Label multipath information MUST be included.IP associated label multipath information MUST be omitted (NULL).
Following subsections describe expected responder LSR
procedures when echo reply is to include DSMAP/DDMAP TLVs,
based on local load balance technique being employed. In case
responder LSR performs deviating load balance techniques per
downstream basis, appropriate procedures matching to each
downstream load balance technique MUST be operated.
Responder MUST set {L=0, E=0} in DS flags.If multipath information type {2, 4, 8} is received,
responder MUST comply with /.If multipath information type {9} is received,
responder MUST reply with multipath type {0}.If multipath information type {10} is received,
responder MUST reply with multipath information type
{10}. "Label Multipath Information" and
"Associated Label Multipath Information"
sections MUST be omitted (NULL). If no matching IP address
is found, then "IPMultipathType" field MUST be
set to multipath information type {0} and "IP
Multipath Information" section MUST also be omitted
(NULL). If at least one matching IP address is found, then
"IPMultipathType" field MUST be set to
appropriate multipath information type {2, 4, 8} and
"IP Multipath Information" section MUST be
included.Responder MUST set {L=0, E=1} in DS flags.If multipath information type {9} is received,
responder MUST reply with multipath type {0}.
If multipath type {2, 4, 8, 10} is received, responder
MUST respond with multipath type {10}. "Label
Multipath Information" section MUST be omitted
(NULL). IP address set specified in received IP
multipath information MUST be used to determine the
returning IP/Label pairs. If received multipath
information type was {10}, received "Label
Multipath Information" sections MUST NOT be used to
determine the associated label portion of returning
IP/Label pairs. If no matching IP address is found, then
"IPMultipathType" field MUST be set to
multipath information type {0} and "IP Multipath
Information" section MUST be omitted (NULL). In
addition, "Assoc Label Multipath Length" MUST
be set to 0, and "Associated Label Multipath
Information" section MUST also be omitted
(NULL). If at least one matching IP address is found,
then "IPMultipathType" field MUST be set to
appropriate multipath information type {2, 4, 8} and
"IP Multipath Information" section MUST be
included. In addition, "Associated Label Multipath
Information" section MUST be populated with list of
labels corresponding to each IP address specified in
"IP Multipath Information"
section. "Assoc Label Multipath Length" MUST
be set to appropriate value.
Responder MUST set {L=1, E=0} in DS flags.If multipath information type {2, 4, 8} is received,
responder MUST reply with multipath type {0}.If multipath information type {9} is received,
responder MUST comply with
/ as updated by
.
If multipath information type {10} is received,
responder MUST reply with multipath information type
{10}. "IP Multipath Information" and
"Associated Label Multipath Information"
sections MUST be omitted (NULL). If no matching label is
found, then "LbMultipathType" field MUST be set
to multipath information type {0} and "Label
Multipath Information" section MUST also be omitted
(NULL). If at least one matching label is found, then
"LbMultipathType" field MUST be set to
appropriate multipath information type {9} and "Label
Multipath Information" section MUST be included.Responder MUST set {L=1, E=1} in DS flags.If multipath information type {2, 4, 8} is received,
responder MUST reply with multipath type {0}.If multipath type {9, 10} is received, responder MUST
respond with multipath type {10}. "IP Multipath
Information" section MUST be omitted (NULL). Label
set specified in received label multipath information MUST
be used to determine the returning Label/Label pairs. If
received multipath information type was {10}, received
"Label Multipath Information" sections MUST NOT
be used to determine the associated label portion of
returning Label/Label pairs. If no matching label is
found, then "LbMultipathType" field MUST be set
to multipath information type {0} and "Label
Multipath Information" section MUST be omitted
(NULL). In addition, "Assoc Label Multipath
Length" MUST be set to 0, and "Associated Label
Multipath Information" section MUST also be omitted
(NULL). If at least one matching label is found, then
"LbMultipathType" field MUST be set to
appropriate multipath information type {9} and "Label
Multipath Information" section MUST be included. In
addition, "Associated Label Multipath
Information" section MUST be populated with list of
labels corresponding to each label specified in
"Label Multipath Information"
section. "Assoc Label Multipath Length" MUST be
set to appropriate value.MS-PW stitching LSR that xconnects flow-aware pseudowires
behaves in one of two ways:
Load balances on previous flow label, and carries over
same flow label. For this case, stitching LSR is to behave as
procedures described in .Load balances on previous flow label, and replaces flow label
with newly computed. For this case, stitching LSR is to behave
as procedures described in .Entropy Label Indicator (ELI) is a reserved label that has no
explicit FEC associated, and has label value 7 assigned from
the reserved range. Use Nil FEC as Target FEC Stack sub-TLV to
account for ELI in a Target FEC Stack TLV.Entropy Label (EL) is a special purpose label with label
value being discretionary (i.e. label value may not be from
the reserved range). For LSP verification mechanics to perform
its purpose, it is necessary for a Target FEC Stack sub-TLV to
clearly describe EL, particularly in the scenario where label
stack does not carry ELI (ex: FAT-PW ). Therefore, this document defines a EL FEC to allow a
Target FEC Stack sub-TLV to be added to the Target FEC Stack
to account for EL.The Length is 4. Labels are 20-bit values treated as numbers.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Label is the actual label value inserted in the label stack;
the MBZ fields MUST be zero when sent and ignored on
receipt.Two flags, L and E, are added in DS Flags field of the
DSMAP/DDMAP TLVs. Both flags MUST NOT be set in echo request
packets when sending, and ignored when received. Zero, one or
both new flags MUST be set in echo reply packets.
DS Flags
--------
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| MBZ |L|E|I|N|
+-+-+-+-+-+-+-+-+
Flag Name and Meaning
---- ----------------
L Label based load balance indicator
This flag MUST be set to zero in the echo request. LSR
which performs load balancing on a label MUST set this
flag in the echo reply. LSR which performs load
balancing on IP MUST NOT set this flag in the echo
reply.
E ELI/EL imposer indicator
This flag MUST be set to zero in the echo request. LSR
which imposes ELI/EL MUST set this flag in the echo
reply. LSR which does not impose ELI/EL MUST NOT set
this flag in the echo reply.
Two flags result in four load balancing techniques which echo
reply generating LSR can indicate:
{L=0, E=0} LSR load balances based on IP and does not
impose ELI/EL.{L=0, E=1} LSR load balances based on IP and imposes
ELI/EL.{L=1, E=0} LSR load balances based on label and does not
impose ELI/EL.{L=1, E=1} LSR load balances based on label and imposes
ELI/EL.One new multipath information type is added to be used in
DSMAP/DDMAP TLVs. New multipath type has value of 10.
Key Type Multipath Information
--- ---------------- ---------------------
10 IP and label set IP addresses and label prefixes
Multipath type 10 is comprised of three sections. One section
to describe IP address set. One section to describe label
set. One section to describe another label set which associates
to either IP address set or label set specified in the other
section.Multipath information type 10 has following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|IPMultipathType| Reserved(MBZ) | IP Multipath Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| (IP Multipath Information) |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|LbMultipathType| Reserved(MBZ) | Label Multipath Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| (Label Multipath Information) |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved(MBZ) | Assoc Label Multipath Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| (Associated Label Multipath Information) |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Multipath Information
This section reuses IP multipath information from
. Specifically, values {0, 2,
4, 8} can be used.Label Multipath Information
This section reuses label multipath information from
. Specifically, values {0, 9}
can be used.Associated Label Multipath Information
"Assoc Label Multipath Length" is a 16 bit field of multipath information which indicates length in octets of the associated label multipath information."Associated Label Multipath Information" is a list of labels with each label described in 24 bits. This section MUST be omitted (NULL) in an MPLS Echo Request message. A midpoint which imposes ELI/EL labels SHOULD include "Assoc Label Multipath Information" in its MPLS Echo Reply message, along with either "IP Multipath Information" or "Label Multipath Information". Each specified associated label described in this section maps to specific IP address OR label described in the "IP Multipath Information" section or "Label Multipath Information" section. For example, if 3 IP addresses are specified in the "IP Multipath Information" section, then there MUST be 3 labels described in this section. First label maps to the lowest IP address specified, second label maps to the second lowest IP address specified and third label maps to the third lowest IP address specified.There are couple of scenarios where LSP path tracing
mechanics are not supported in this draft revision.
When one or more LSP transit node(s) performs label based
load balancing on a label that is not bottom-of-stack label
when Entropy Label Indicator is not included.When one or more LSP transit node(s) performs label based
load balancing on a label other than Entropy Label when
Entropy Label Indicator and Entropy Label pair is included.Beyond those specified in ,
and , there
are no further security measured required.DS flags ... not maintained by IANA. Should it be?Multipath information types ... not maintained by
IANA. Should it be?IANA is requested to assign a new sub-TLV from the
"Sub-TLVs for TLV Types 1 and 16" section from
"TLVs" sub-registry within the "Multi-Protocol
Label Switching (MPLS) Label Switched Paths (LSPs) Ping
Parameters" registry.Following value appears to be next available sub-TLV
value. Requesting IANA to allow specified value as early
allocation.
Value Meaning Reference
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26 Entropy Label FEC this document
TBDNagendra Kumar
Cisco Systems
Email: naikumar@cisco.com